This eliminates the error messages that would be displayed/logged when
attempting the SYS_capset SYSCALL, which fails when not root.

So the capabilities dance enabled with USE_LINUX_CAPS (which is automatically
enabled when /usr/include/sys/capabilites.h exists, which is installed with
the libcap2-dev package) apparently is still useful if you need to start
sbbs as root: the main thread will remain as root even when the child
threads have their user-id's changed to the user specified in the [UNIX]
section of sbbs.ini or on the command-line
- so reducing the enabled privilege set for this root/main thread to the
minimum needed, is a good security measure. It does this reduction (call to
linux_minprivs() after the call to change_user()) so I'm not exactly sure how
that works, but according to Deuce, this is what's happening. :-/